The present invention relates to implantable stimulation devices, e.g., cochlear prosthesis used to electrically stimulate the auditory nerve, and more particularly to an electrode array for use with a cochlear stimulator that is designed to hug the modiolus so as to place electrode contacts of the electrode array in close proximity to the ganglion cells and thereby to the auditory nerve fibers.
Hearing loss, which may be due to many different causes, is generally of two types: conductive and sensorineural. Of these, conductive hearing loss occurs where the normal mechanical pathways for sound to reach the hair cells in the cochlea are impeded, for example, by damage to the ossicles. Conductive hearing loss may often be helped by use of conventional hearing aids, which amplify sound so that acoustic information does reach the cochlea and the hair cells. Some types, of conductive hearing loss are also amenable to alleviation by surgical procedures.
In many people who are profoundly deaf, however, the reason for their deafness is sensorineural hearing loss. This type of hearing loss is due to the absence or the destruction of the hair cells in the cochlea which are needed to transduce acoustic signals into auditory nerve impulses. These people are unable to derive any benefit from conventional hearing aid systems, no matter how loud the acoustic stimulus is made, because their mechanisms for transducing sound energy into auditory nerve impulses have been damaged. Thus, in the absence of properly functioning hair cells, there is no way auditory nerve impulses can be generated directly from sounds.
To overcome sensorineural deafness, there have been developed numerous cochlear implant systemsxe2x80x94or cochlear prosthesisxe2x80x94which seek to bypass the hair cells in the cochlear (the hair cells are located in the vicinity of the radially outer wall of the cochlea) by presenting electrical stimulation to the auditory nerve fibers directly, leading to the perception of sound in the brain and an at least partial restoration of hearing function. The common denominator in most of these cochlear prosthesis systems has been the implantation into the cochlea of electrodes which are responsive to suitable external source of electrical stimuli and which are intended to transmit those stimuli to the ganglion cells and thereby to the auditory nerve fibers.
A cochlear prosthesis operates by direct electrical stimulation of the auditory nerve cells, bypassing the defective cochlear hair cells that normally transduce acoustic energy into electrical activity in such nerve cells. In addition to stimulating the nerve cells, the electronic circuitry and the electrode array of the cochlear prosthesis performs the function of the separating the acoustic signal into a number of parallel channels of information, each representing the intensity of a narrow band of frequencies within the acoustic spectrum. Ideally, each channel of information would be conveyed selectively to the subset of auditory nerve cells that normally transmitted information about that frequency band to the brain. Those nerve cells are arranged in an orderly tonotopic sequence, from high frequencies at the basal end of the cochlear spiral to progressively lower frequencies towards the apex. In practice, this goal tends to be difficult to realize because of the anatomy of the cochlea.
Over the past several years, a consensus has generally emerged that the scala tympani, one of the three parallel ducts that, in parallel, make up the spiral-shaped cochlea, provides the best location for implantation of an electrode array used with a cochlear prosthesis. The electrode array to be implanted in this site typically consists of a thin, elongated, flexible carrier containing several longitudinally disposed and separately connected stimulating electrode contacts, perhaps 6-30 in number. Such electrode array is pushed into the scala tympani duct to a depth of about 20-30 mm via a surgical opening made in the round window at the basal end of the duct. During use, electrical current is passed into the fluids and tissues immediately surrounding the individual electrical contacts in order to create transient potential gradients that, if sufficiently strong, cause the nearby auditory nerve fibers to generate action potentials. The auditory nerve fibers arise from cell bodies located in the spiral ganglion, which lies in the bone, or modiolus, adjacent to the scala tympani on the inside wall of its spiral course. Because the density of electrical current flowing through volume conductors such as tissues and fluids tends to be highest near the electrode contact that is the source of such current, stimulation at one contact site tends to activate selectively those spiral ganglion cells and their auditory nerve fibers that are closest to that contact site. Thus, there is a need for the electrode contacts to be positioned as close to the ganglion cells as possible. This means, in practice, that the electrode array, after implant, should preferably hug the modiolar wall, and that the individual electrodes of the electrode array should be positioned on or near that surface of the electrode array which is closest to the modiolar wall.
In order to address the above need, it is known in the art to make an intracochlear electrode array that includes a spiral-shaped resilient carrier which generally has a natural spiral shape so that it better conforms to the shape of the scala tympani. See, e.g., U.S. Pat. No. 4,819,647. The ""647 U.S. patent is incorporated herein by reference. Unfortunately, while the electrode shown in the ""647 patent represents a significant advance in the art, there exists lack of sufficient shape memory associated with the electrode to allow it to return to its original curvature (once having been straightened for initial insertion) with sufficient hugging force to allow it to wrap snugly against the modiolus of the cochlea.
It is also known in the art, as shown in applicant""s prior patents, U.S. Pat. Nos. 5,545,219 and 5,645,585, to construct an electrode carrier from two initially straight members, a rodlike electrode carrier and a flexible rodlike positioning member. As shown in these patents, the two members extend in substantially parallel relation to and closely alongside each other, but are connected to each other only at their respective leading and trailing end regions. After implant, a pushing force is applied to the positioning member so that it is forced to assume an outwardly arched configuration relative to the electrode carrier, thereby forcing the electrode carrier into a close hugging engagement with the modiolus, thereby placing the electrode contacts of the electrodes in as close a juxtaposition to the cells of the spiral ganglion as possible. The ""219 and ""585 U.S. patents are also incorporated herein by reference.
Unfortunately, while the electrode array taught in the above-referenced ""219 and ""585 patents has the right idea, i.e., to force the electrode carrier into a close hugging engagement with the modiolus, it does so only by use of an additional element that makes manufacture of the lead more difficult and expensive, and only through application of an additional pushing force which is applied to an electrode structure after it is already fully inserted into the cochlea. Such additional pushing force may easily cause damage to the delicate scala tympani. Moreover, the entire electrode array may twist during the insertion process, or when the additional pushing force is applied, thereby causing the electrode contacts to twist and/or be forced away from the modiolus, rather than in a hugging relationship therewith.
Thus, while it has long been known that an enhanced performance of a cochlear implant can be achieved by proper placement of the electrode contacts close to the modiolar wall of the cochlea, two main problems have faced designers in attempting to achieve this goal. First, it is extremely difficult to assemble electrode contacts on the medial side of the an electrode array, facing the modiolus of the cochlea. Second, heretofore there has either been the need for application of an external (and perhaps unsafe) force, or a lack of sufficient shape memory, to allow the electrode (after initial straightening to facilitate insertion) to assume or return to the desired curvature needed to place the electrodes against the modiolar wall so that the curvature wraps snugly around the modiolus of the cochlea. As a result, the electrode contacts of the prior art electrodes are generally positioned too far way from the modiolar wall.
It is thus evident that improvements are still needed in cochlear electrodes, particularly to facilitate assembling an electrode so that the electrode contacts are on the medial side of the electrode array, and to better assure that the electrode assumes a close hugging relationship with the modiolus once implantation of the electrode has occurred.
The present invention addresses the above and other needs by providing an electrode system that allows for correct positioning of the electrode contacts against the modiolar wall of the cochlea. Such xe2x80x9ccorrectxe2x80x9d positioning is achieved through the use of an electrode system that includes the following three main components: (1) an electrode array, made in a straight or slightly curved shape, made on a flexible carrier so that it can easily bend within the cochlea; (2) a flexible positioner, typically molded from a silicone polymer so as to make it easy to slide into the cochlea, and made to assume a curved shape to facilitate its insertion into the cochlea; and (3) a guiding insert or guide tube, made from a biocompatible material, such as platinum (Pt), titanium (Ti) or Teflon, for facilitating the insertion of either the electrode array or the flexible positioner.
In one embodiment, insertion of the electrode array is performed in three main steps. First, the flexible positioner is inserted through the appropriate dimension of cochleostomy. This means it is inserted into the scala tympani (one of the channels of the cochlea) to the desired depth. The desired depth typically involves a rotation of about 360 degrees and causes the positioner to rest against the outer or lateral wall of the scala tympani, leaving an opening slightly larger than the cross-section of the electrode array adjacent the inner wall of the scala tympani. Advantageously, the super-flexible nature of the positioner prevents it from causing damage to the cochlear structure. At the same time, once inserted, it provides a guide for the electrode, and protects the cochlear walls from being damaged or touched directly by the stiffer electrode body.
Second, after insertion of the positioner to the desired depth, the guiding insert may be pushed into the opening of the cochlear.
Third, the electrode array is inserted through the opening of the guiding insert to the desired depth. This desired depth is preferably beyond the depth of the positioner. The distal end of the array advantageously includes one or more engaging or locking barbs or teeth that engage with corresponding barbs or teeth at the distal end of the positioner. At this stage, the electrode is positioned very close to the modiolus of the cochlea. Then, as a final optimization of the position of the electrode contacts of the electrode array, the electrode array is pulled back slightly (about 2 mm). This backward motion assures that the distal tips of the electrode array and the positioner are engaged by the barbs located thereon. Such engagement may further serve to force the electrodes into direct contact with the modiolar wall.
In another embodiment, a similar procedure is followed, except that the electrode array is inserted into the cochlea first, and then the positioner is inserted through the guide tube so as to lie along a back side (i.e., the side opposite the electrode contacts) of the electrode array, thereby positioning the electrode contacts of the array near the modiolar wall. In such embodiment, the positioner advantageously has at least one pair of guide tabs, or wings, at or near its distal tip, so as to keep the distal tip of the positioner from slipping off of the distal tip of the electrode array. In other words, the tabs or wings at the distal tip of the positioner, form a channel, or groove, through which the body of the electrode array may slide as the positioner is inserted behind the electrode array, and which once inserted, keep the positioner distal tip in a desired position alongside (and, more particularly, along a back side of) the distal tip of the electrode array. In such embodiment, the positioner also may include a pair of flexible side walls near or at its proximal end. The space between such side walls also forms a channel or groove into which the body of the electrode array may be positioned as the positioner is slid into position alongside the electrode array within the cochlea. Such channel or groove may further include a sloping floor that acts as an additional spacer that pushes or forces a proximal end of the electrode array into close engagement with the basal end of the cochlea as the positioner is fully inserted into the cochlea.
In accordance with an alternate embodiment of the invention, an electrode positioner is provided that may be used with almost any electrode array that is to be inserted into the cochlea in order to assure that a desired modiolar-hugging position is achieved with the electrode contacts of the array.
In accordance with yet an additional embodiment of the invention, a cochlear electrode system is provided that includes (1) an electrode array and (2) an electrode positioner. Using a preferred insertion technique or method, the electrode array is first inserted into the cochlea as far as it reasonably can be; then the positioner is inserted into the cochlea, behind the electrode array so as to force or push the electrode contacts of the array against the modiolar wall. As required, a guide tube may be used to assist with inserting the electrode positioner into the cochlea. Moreover, as the positioner is thus inserted into the cochlea behind the electrode array, the positioner carries the electrode deeper into the cochlea, e.g., approximately xc2xd turn deeper. In such instance, the positioner need not be equipped with internal barbs at its distal end, but it may be.
Advantageously, the electrode system of the present invention achieves the following goals: (1) it helps assure that the electrode array is optimally positioned against the modiolar wall in a cochlea of any size; (2) the insertion of the electrode array avoids or produces minimal trauma to the cochlear structure; (3) it allows deep insertion beyond 360 degrees; (4) it can be manufactured using easy, low cost technology; and (5) the electrode and/or the positioner can be easily removed and reinserted, if required.